Dicyclopentadiene (DCPD) is an item of commerce and is produced in the steamcracking of gas oils, naphthas, and other hydrocarbons. It is usually obtained as a by-product from the steamcracker effluent after distillation and heat-soaking of the effluent. Unfortunately, the crude DCPD produced by steam cracking is typically of low purity which is unacceptable for metathesis polymerization.
Various methods have been used to purify the crude DCPD. One such purification method involves the distillation of crude DCPD to remove low boiling compounds such as butadiene, isoprene, pentadienes, and cyclopentadiene (CPD). This is followed by a second stage distillation or is done as a side stream operation to provide a higher purity DCPD.
This distillation method has the disadvantage of low recovery of DCPD of suitable quality for metathesis polymerization. Yields are typically 70% or less. Additionally, it has the disadvantage of being susceptible to oxygen leaks. These leaks give rise to the formation of oxygenated compounds which inhibit metathesis polymerization.
Another method of DCPD purification is to thermally crack DCPD and some of the codimers of cyclopentadiene (CPD) and C.sub.4 and C.sub.5 acyclic dienes in (1) a kettle-type reboiler (e.g., a shell and tube heat exchanger); (2) a vapor phase cracker; and (3) a thermo-syphon reboiler with an inert hydrocarbon diluent to reduce fouling. The monomers which are the effluent of these systems can then be separated via conventional distillation to provide a monomer concentrate highly enriched in CPD. Controlled dimerization of this CPD stream can afford DCPD in concentrations of greater than 98%.
This thermal cracking/dimerization method does not give directly a DCPD of purity suitable for metathesis polymerization. The DCPD must be fractionally distilled to remove the aforementioned low-boiling C.sub.4 and C.sub.5 acyclic dienes and CPD. Long dimerizer residence times at elevated temperatures (i.e., greater than 83.degree. C.) can give rise to the formation of codimers of CPD and C.sub.4 and C.sub.5 acyclic dienes. Some of these codimers, such as tetrahydroindene and 6-methyltetrahydroindene, are known to inhibit metathesis polymerization. Additionally, the dimerization conditions and/or distillation conditions can give rise to the formation of CPD trimers. These trimers may polymerize at rates different from DCPD and may alter the structural properties of the poly(DCPD).
The present inventors have developed a unique process which overcomes the low purity problems associated with conventional distillation methods and the formation of codimers which results from the conventional thermal cracking/dimerization methods discussed above. In either case, the resultant dimerization product inhibits metathesis polymerization which is a highly desirable commercial application for dicyclopentadiene.
The present invention also provides many additional advantages which shall become apparent as described below.